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MOLKCULAk   ROTATION                       119

illumination in the two halves of the field, the nieol N must
be turned through an angle equal to the angle of rotation, which
is then measured on the divided circle.

When the angle  is small, i.e. when the plane of
vibration of the polarised light is almost parallel to the optic
axis of the quartz, the greatest degree of sensitiveness is
attained, for then a very small change in the position of N
causes a great difference in the respective illuminations in the
two halves of the field. As  increases, the sensitiveness
diminishes, but a greater total intensity of illumination is ob-
tained. By moving j (Fig. 71) the position of the nicol p may
be altered. For clear colourless liquids the angle a may be
made comparatively small ; but in the case of coloured liquids
it is necessary to have a larger, and so obtain a greater intensity
of light at the cost of sensitiveness.

Calculation of Results ; Homogeneous Liquids.-
The angle of rotation, represented by i> (for sodium light), varies
with the length of the column of substance through which the
light passes. One decimetre has been chosen as unit of length.
The angle also varies with the temperature, which must conse-
quently be determined for each observation.

For the comparison of the rotary power of different substances,
use is made of the constant specific rottition, which may be defined
as the angle of rotation, produced by I gram of active substance
in I c.c. by a layer I dm. in length. This is obtained by dividing
the observed angle of rotation by the product of the length in
decimetres, and the density of the substance at the temperature
at which the observation was made.

Molecular Rotation is the above quantity multiplied by
the molecular weight M of the compound, and divided by 100 to
avoid unwieldy numbers, and is represented thus .........

It expresses the angle of rotation of i mm. of active substance
containing i gram-molecule in i c.c.